Apparatus for the treatment of articles by high velocity impacting thereof with a particulate abrasive material

ABSTRACT

A method is provided for soldering using tin-containing solder to substrate surfaces that are gold plated wherein a selected portion of the gold plated surface is treated by impacting with a high velocity stream of fine nickel containing abrasive particles to remove all the gold from the treated surface and then effecting joining to the treated surface by conventional soldering techniques.

FIELD OF THE INVENTION

The present invention relates to soldering as a means of joining to goldplated substrates and, more particularly, to a method and apparatus forjoining by soldering to gold plated substrates including elements ofelectronic components and devices and the like such as "leads,"terminals (terminations), etc. using tin-containing solder.

BACKGROUND OF THE INVENTION

There are many applications in which electronic components, devices andthe like have a variety of parts including "leads," contacts and similarconnecting elements which are plated with a thin layer of gold (about 50microinches or even more) to enhance the electrical quality andenvironmental resistance of the circuits involved. Electrical qualityand reliability of the various elements and/or components in thecircuits play a major role in the suitability thereof and of the devicesand systems in which they are employed. Quality and reliabilityfrequently depend on the type of connecting elements used and the meansused to join them.

Soldering is a widely used technique for readily joining elements ofelectrical circuits in both mass production and hand assemblyoperations. The structure and composition of the elements, such as the"leads," to be joined as well as the type, size and mode of operation ofthe circuit and components thereof are factors in determining thesuitability of known soldering techniques and materials. For example,the strength and reliability of the joint frequently depends on both thecomposition of the elements to be joined and of the joining materialsuch as the solder; the type and/or size of elements in an electroniccomponent must be considered when establishing the time and temperaturesthat ca be employed in the joining operation; service conditions andaccessibility relate to the materials and temperatures required for thejoining techniques and suitability thereof in mass productionoperations.

A widely used soldering technique for electronic components employstin-containing solder whereby joining can be carried out rapidly atrelatively low temperatures, e.g. about 400° F. However, as is known,joining with a tin-containing solder to surfaces over which even a flashcoating (about 10 to 30 microinches) of gold has been applied results inembrittlement and ultimate failure of the gold-tin solder joint over aperiod of time by virtue of the interaction of the gold and tin. Thus,the strength, time and temperature advantages of this type of joiningtechnique are not readily available for a large number of electroniccomponents where gold plating of the surfaces is required.

Numerous attempts have been made over the years to overcome the problemof soldering to gold plated surfaces, but to date no routine oreconomically feasible procedures and/or solder materials have beendeveloped which afford the strength, reliability and other advantages ofbasic tin-containing solders. Moreover, in those cases where the use ofhigh temperatures would be detrimental to the electronic components orsubstrates, it has been necessary to employ complicated and costlymechanical or chemical procedures for joining to those surfaces whichare required to be gold plated. For example, one such procedure involvesmasking portions of the part before gold plating to limit the coating toareas where joining to the surface would not be needed. Masking and thenremoval of the masking before assembly and related operations greatlyincrease the complexity and costs, particularly when mass production oflarge quantities of small parts is involved.

Mechanical abrading, reforming, compacting and cleaning of articles madeof metals, ceramics, refractory materials or plastics by "sand-blasting"and analogous high velocity impacting treatment procedures are wellknown and widely used. This manner for treatment of generally hardsurfaces typically involves bombardment of them with fine grains ofabrasive materials at high velocity utilizing a jet of compressed air orother suitable fluid. While the term "sandblasting" has generallyattained a generic meaning for treating the surfaces of articles by highvelocity impacting with granular or particulate abrasive materials, thetreatments used involve a wide range of different apparatus andprocedures as well as many different types, sizes and shapes of abrasivematerials depending on the application, such as the type and compositionof the materials being treated, and the purpose of the treatment.

Quartz sand used for blasting parts in the past with the aid ofcompressed air, in general, has been found to be unsuitable forenvironmental reasons as well as being subject to premature breakingwhen impacting metal surfaces. In U.S. Pat. Nos. 2,145,756, 2,336,001,2,362,310, 2,600,358, 2,670,281, 2,773,339, 2,796,338, 3,188,776,4,035,962, 4,289,541 and Re. 31,883 are disclosed examples of some ofthe many metallic materials used as particulate abrasives for "sandblasting" and grinding treatment of the surfaces of metal articlesincluding slabs, sheets and castings. None of these patents deal withthe problem of joining to surfaces to be or that have been gold plated,or to apparatus for the preparation of parts used in electroniccomponents.

More recently, "sand blasting" and analogous techniques have been usedfor a variety of non-metal treating applications including, for example,removing insulation from insulated wires as disclosed in U.S. Pat. No.3,636,623, and as a finishing step for plastic parts such as removingflash or excess material from molded parts including electroniccomponents as disclosed in U.S. Pat. Nos. 4,545,155, 4,555,876 and4,561,219. In such applications, special apparatus and fixtures are usedfor controlling the area treated with various different types ofabrasive materials, typically of special composition, size and shape.Here again, the problem of joining to gold plated surfaces is not dealtwith, nor is there a suggestion of any method which may be suitable,although the treatment of substrate surfaces to be coated and thefinishing of coated surfaces of electronic components are disclosed.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method for readilyjoining by soldering to gold plated surfaces using tin-containingsolders which is efficient and results i strong, reliable solder jointsusing mass production and manual operations.

It is a further object of the present invention to provide a method forreadily joining by soldering to gold plated surfaces of elements inelectrical circuits of electronic components using tin-containingsolders, wherein the tin containing solder joint is strong and reliableand can be effected by mass production operations.

It is a still further object of the present invention to provide amethod for readily soldering to the gold plated surface of an electroniccomponent using a tin-containing solder by removing gold plate from onlythe portion of the surface of the component where joining is to beeffected without disturbing the electrical quality achieved by the goldplate on the surface of the electronic component.

It is another object of the present invention to provide a method fortreating gold plated surfaces of electronic components and the like byhigh velocity impacting with particulate abrasive materials to effectremoval of the gold plate from a selected portion of the surface of thecomponent to permit the preparation of strong, reliable solder jointswith the surface of the component using a tin-containing solder.

It is still another object of the present invention to provide anapparatus for the treatment of a selected portion of the gold platedsurface of an article, preferably the removal of gold plate from thesurface of the article, by the high velocity impact with a particulateabrasive material, which includes a support for the article to betreated, a masking for the article outlining the particular portion ofthe surface of the article to be treated, and a manifold chamberspacedly mounted over the support for said article which is adaptablefor receiving a supply of particulate abrasive material and admixingtherewith a fluid suitable for the high pressure and velocity dispensingof said abrasive material, said manifold chamber having at least oneinlet port for the ingress of high pressure fluid such as a compressedgas into said manifold chamber and at least one dispensing nozzle forsaid abrasive material disposed in said manifold chamber in verticalregister with the portion of the surface of the article to be treated.

Still another object of the present invention is to provide aparticulate abrasive material for the high pressure and velocity impacttreatment of hard surfaces, such as the gold plated surface of articles,by which the required treatment thereof may be effected, such as removalof the gold plate from selected portions of the articles surface, andwhereby strong, reliable solder joints may be obtained directly with thetreated surface using tin-containing solder without the need for furthertreatment that attends the use of other known impacting materials.

In accordance with the present invention there i provided a method forjoining to the gold plated surface of an article by soldering with atin-containing solder which comprises: providing a supported substrateor member such as an article with a gold plated surface, e.g. anelectrical lead for an electronic component; effecting the removal ofsaid gold plate from a selected portion of the surface of saidsubstrate, preferably substantially only the area to which joining bysoldering is intended, by impacting said supported gold-plated substratewith a high pressure stream of fluid-borne, preferably airborne,abrasive material comprising nickel or nickel-containing particles; andeffecting joining to the gold-free portion of the surface of saidsubstrate by soldering thereto with a tin-containing solder.

Soldering to the surface of the article in the area where gold plate hasbeen removed can be readily carried out using conventional solderingtechniques (time and temperature). Surprisingly and unexpectedly it hasbeen found that a strong, reliable solder joint will be directly formedwith the substrate without detracting from the appearance and electricalor environmental advantages achieved by gold plate covering the otherportions of the substrate or article. In accordance with the practice ofthe invention, for example, gold plated lead frames prepared with alarge number of electrical "leads" by conventional methods, can bereadily treated for removal of the gold plate from a selected portion ofeach of the "leads" and then joined to elements of the electricalcircuit by conventional tin-containing soldering techniques. The methodof the invention may be employed with elements of miniature electroniccircuits and components and may be effected by manual or mass productionoperations.

In another aspect of the invention, there is provided an apparatus ordevice for treating the gold plated surface of an article by the highpressure impacting thereof with a particulate abrasive material whichcomprises: support base means for an article to be treated, preferably aflat, thin self-supportable sheet member; masking means adapted to bedisposed over the supported article to be treated with at least oneselected portion of the surface of the article exposed; and manifoldchamber means adaptable to receive a quantity of particulate abrasivematerial and to effect admixture thereof with a fluid under highpressure, preferably compressed air, said manifold chamber meansincluding at least one connector means for ingress of a fluid under highpressure, at least one nozzle means for dispensing at high pressure andvelocity a fluid-borne particulate abrasive material from within saidchamber and means for engaging said masking means about a portion of thesurface of the article to be treated, said manifold chamber means beingmovably mounted over said base support means and spaced therefrom withsaid nozzle means and said means for engaging said masking means insubstantial register with the portion of the surface of the article tobe treated.

The apparatus of the invention can be readily used to treat the surfaceof a flat article such as a lead frame having a plurality of electricalleads interconnected for initial handling. Such lead frames, which havethe surface(s) thereof gold plated to meet electrical requirements, canbe treated using the apparatus of the invention to remove gold platingfrom selected portions of their surface by blasting with abrasiveparticles in preparation for the joining thereof by soldering with atin-containing solder.

In still another aspect of the present invention there is provided ametallic abrasive and blasting material for treating hard surfaces bybeing continually hurled at high velocity against such surfacescomprising selectively sized fine particles of nickel ornickel-containing alloys such as nickel-chromium and nickel-vanadium,wherein nickel is the primary component of such alloys. It has beensurprisingly and unexpectedly discovered that such nickel ornice-containing particles possess a distinctive combination ofproperties, including high density, configuration, hardness, andbreak-down resistance which enable it to be used with effectiveness as asurface impact treating agent for many substrates and materials inconjunction with soldering and the like operations which may be used tojoin to the treated surfaces.

Other objects and advantages of the present invention will becomeapparent from the detailed description thereof taken in conjunction withthe drawings

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating one aspect of the invention, there isshown in the accompanying drawing one form which is presently preferred;it being understood that the invention is not intended to be limited tothe precise arrangements and instrumentalities shown.

FIG. 1 is a perspective view of a gold plated "lead frame" showing anarticle of the type which ca be treated according to the presentinvention;

FIG. 2 is an enlarged and fragmentary sectional view taken along theline 2--2 of FIG. 1;

FIG. 3 is a perspective view of the lead frame article of FIG. 1 aftertreatment of selected portions of the surface thereof in accordance withthe process of the invention and to which joining by soldering can beeffected;

FIG. 4 is an enlarged and fragmentary sectional view taken along theline 4--4 of FIG. 3;

FIG. 5 is a perspective view of an abrasive blasting apparatus inaccordance with the invention;

FIG. 6 is an exploded perspective view of the apparatus of FIG. 5;

FIG. 7 is a perspective view illustrating the manifold chamber of theapparatus of FIG. 5 taken along the line 7--7 of FIG. 6;

FIG. 8 is a partial exploded perspective view of the apparatus of FIG.5;

FIG. 9 is an enlarged sectional view taken along line 9--9 of FIG. 5;and

FIG. 10 is an enlarged and fragmentary perspective sectional view takenalong line 10 of FIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is predicated upon the discovery that joining togold plated surfaces of various articles such as components used inelectrical circuits can be readily effected by conventional solderingtechniques using tin-containing solders when the gold plate iscompletely removed from selected portions of the surface of thesubstrate by high velocity impact treatment with particulate nickel andnickel-containing alloys abrasives.

Referring now to the drawing, where like reference numerals identifylike parts, there is illustrated in FIGS. 1 and 2 a metal plate showngenerally as 10, which is comprised of an array of a plurality of spacedelectrically conductive leads 12 formed therein, which plate isconventionally referred to as a "lead frame." The leads 12 are intendedfor use as elements of the circuitry of electronic components, forexample, PILL type stripline terminations (not shown). During themanufacture of such components, selected lead elements 12 of the arrayin the frame 10 will be joined at designated points of the electriccircuit, following which the plate is cut to separate the free ends ofthe joined leads 12 therefrom resulting in a finished component.Intermediate the array of leads 12 are spaced holes 38 which may serveto position and hold the "lead frame" 10 during treatment of the surfaceand/or assembly of the components.

The lead frame 10 includes a copper metal base or substrate 18 overwhich is plated a thin barrier layer of nickel 16 and then a thin layerof gold 14 (at least about 50 microinches). Gold plating of electroniccomponents is frequently required to insure the electrical quality andreliability of the circuitry for extended periods of storage and use. Asindicated, it is well known that when soldering by conventionaltechniques with tin-containing solder to surfaces on which even traceamounts of gold may be present, interaction of the gold and tin resultsin the ultimate embrittlement and failure of the tin-gold joint.

In accordance with the invention there is provided a method for treatingthe surface of gold plated articles such as leads 12 arrayed in a leadframe 10 to readily permit soldering to the surface of such articleswith tin-containing solder which comprises: supporting the article to betreated such as the lead frame 10 which has a gold plated surface; andeffecting the removal of the gold plate from selected portions of asurface of the supported lead frame, preferably only the area of thesurface of the leads 12 to which joining by soldering is intended, byimpacting said supported lead frame 10 with a high pressure stream offluid-borne, preferably airborne, abrasive material comprising particlesof nickel, nickel-containing alloys and mixtures of the same, preferablywhile the lead frame is held securely and accurately positioned relativeto the pressurized stream of abrasive particles.

In FIGS. 3 and 4 there is illustrated the lead frame 10 which has beentreated in accordance with the practice of the invention by highpressure impacting with particulate abrasive materials to the extentthat the layer of gold plate 14 has been completely removed fromselected portions of one surface 15 of the leads 12 together with aportion of the nickel plate coating 16 on the copper substrate 18. Theuntreated gold plated surfaces of the leads 12 and lead frame 10 are notchanged in performance or appearance. Joining to the treated surface 15of the leads 12 by soldering with a tin-containing solder may then bereadily effected to form a strong joint therewith without any detrimentto the untreated surfaces of the leads or embrittlement of the solderjoint o the surface of the leads.

According to the present invention, in order to treat the surfaces ofgold plated substrates such as electrical leads without detriment to theelements or to the treated surfaces, yet effecting complete removal ofthe gold plate from selective portions of the surface without anydetrimental residue clinging thereto, it is important that nickelcontaining particles or "powder" of appropriate grain size be used asthe abrasive medium for the surface treatment. Generally speaking, astream of nickel or nickel alloy particles are projected upon the workpiece at high velocity in a blasting technique analogous to "sandblasting" and impacting by the abrasive particles removes gold platefrom the treated surfaces.

The abrasive particles are propelled at high velocity by a fluid stream,preferably compressed gas or air at about 30 to 90 psi. Nickel powder orparticles, powder or particles of nickel alloys such as nickel-copper,nickel-chromium, nickel-vanadium and the like wherein nickel is theprimary component, or mixtures thereof are satisfactory for the purposesof the invention. In general, the nickel and nickel-containing particlesor powder are used directly, i.e., not crushed and used as grit.

While the size of the nickel particles or powder and the amount thereofin the fluid stream useful in accordance with the invention is notcritical and generally depends on the type and size of the substrate andthickness of gold plate, for purposes such as treating the surface oflead frames 10, extremely fine nickel particles or powder in the sievesize range of 200 to 325 (0.074 mm to 0.044 mm) are especiallyadvantageous. In general, the fluid stream will contain about 90% to 95%by weight of the abrasive particles and about 5% to 10% by weight ofcarrier.

The length of time required for treatment of a surface with a highvelocity stream of abrasive particles in accordance with the inventionis generally quite short, e.g., 40 seconds or less, and may be readilydetermined by routine experimentation. After treatment in accordancewith the invention of selected portions of the surface of gold platedsubstrates, all of the gold plate has been removed without undesirableresidue of the abrasive material remaining and the treated surfaces arein condition for directly forming a strong solder joint therewith usinga tin-containing solder such as 95:5 tin/silver and 60:40 tin/lead. Suchsolder joints are not subject to embrittlement by virtue of the reactionof gold and tin, nor are adjacent untreated gold plated surfacescompromised as to long term electrical and environmental quality orappearance.

Referring now to FIGS. 5-10, there is illustrated an abrasive blastingapparatus or device, shown generally as 20, in accordance with thepresent invention for performing the high velocity impact treatment ofgold plated articles such as lead frames 10 which comprise a pluralityof leads 12. The apparatus 20 includes a support base 22 having anarticle support plate 26 fastened over a base plate 24 by conventionalmeans such as screws 25 or the like and a manifold chamber 40 which isremovably mounted on the support base 22 in a predetermined spacedrelationship. All parts of the apparatus 20 can be fabricated fromstainless steel or other abrasion resistant metal.

The article support plate 26 has a planar top surface 27 large enough tosupport thereon a flat article or workpiece to be treated, such as a"lead frame" 10, and a mask 32 for defining the surface of the leadframe to be treated. Mask 32 is a thin metal sheet fabricated fromstainless steel or the like having an array of spaced cut-outs 34therethrough which is designed to be supported on the support plate overthe work piece 10. Outwardly extending from the top surface 27 ofsupport plate 26 are two sets of spaced alignment pins, 28 and 30, whichare secured to support plate 26. Spaced alignment pins 28 serve toposition and hold the lead frame 10 with mask 32 superimposed thereoverupon the surface 27 of support plate 26 by virtue of being in registerwith holes 38 through the frame 10 and holes 36 through the mask 32.Such pins 28 are of a length suitable to hold the superimposed leadframe 10 and mask 32 thereon yet not interfere with the spacedpositioning of a manifold chamber 40 thereover. Elongated, spacedalignment pins 30 serve to position and support the manifold chamber 40relative to the lead frame 10, the mask 32, and the support base 22during operation of the apparatus.

Manifold chamber 40 is comprised of a frame 44, a top plate 46 and agenerally planar bottom plate 48 which are secured together byconventional means such as screws 43 to enclose a pressure chamber 54within which a quantity of particulate abrasive material and gas underhigh pressure may be suitably retained and mixed. Spaced holes 42through the top plate 46, the opposing short ends of the frame 44 andthe bottom plate 48 are aligned when manifold chamber 40 is assembled asshown and, as indicated, are in register with the elongated alignmentpins 30 extending from the support base 22. Secured to and extendingthrough the top plate 46 of the manifold chamber 40, are inlet anddispersing connectors 50 for the supply of a mixture of compressed gasand abrasive particle 70 into pressure chamber 54 which may also be usedas inlets for a supply of abrasive particles 70 and/or compressed gasinto pressure chamber 54. Such compressed gas, abrasive particles and/ormixtures thereof are supplied through supply lines 52 from conventionalsources (not shown) during operation of the apparatus.

Located within the base plate 48 of manifold chamber 40 and extendingtherethrough are a plurality of spaced nozzle assemblies 60 fordispensing streams of abrasive particles from within the pressurechamber 54. Each of the nozzle assemblies 60 comprises a plurality ofspaced orifices 62, each orifice 62 being in general alignment with aportion of the surface of the workpiece to be treated, thus serving tomore uniformly distribute the high velocity streams of abrasiveparticles issuing from the manifold chamber 40 over the plurality ofsurfaces of the lead frame 10 to be treated. The diameter of theorifices 62 may vary depending, for example, on the particle size of theabrasive materials, the size of the surface to be treated, the thicknessof gold plate, etc. In general, orifices of a diameter ranging fromabout 0.015" to 0.040" may be advantageously employed. It would beevident that the type, location and construction of the nozzleassemblies may vary depending on the type and size of article to betreated as well as the particular construction of the manifold chamberused.

Also secured to the bottom plate 48 of manifold chamber 40 are aplurality of spaced, parallel, elongated rods 64 with the free endsthereof extending outwardly a substantially equal length. The rods 64extend from the bottom plate 48 in a series of arrays 66, with eacharray 66 thereof generally located within the periphery defined by anozzle assembly 60 in the bottom plate, but somewhat outside theperiphery defined by the orifices 62 in each of the nozzle assemblies 60and the periphery of each cut-out 34 in the mask 32. As shown, the rods64 serve to position the manifold chamber 40 in generally uniformlyspaced relation 72 to the support base 22 as well as securing the edgesof the cut-out portions 34 of the mask 32 against the work piece 10 andthe support base 22. The length of the rods 64 and ,thus, the spacingmay vary depending on the application, size of the abrasive particles,the pressure and velocity of the fluid used. In general spacing 72 whichis close enough for application of the necessary impact energy to thesurface being treated yet permitting the ready removal of spent abrasiveparticles from the article being treated and from the nozzle exits ispreferred and typically may advantageously range from about 1/8" to3/8", or even greater.

In FIGS. 6 and 8-10 are shown a preferred method of treating an articlesuch as a lead frame 10 in accordance with the practice of the inventionemploying an embodiment of the apparatus of the invention 20. Assemblyof the blasting apparatus 20 with manifold chamber 40 mounted onalignments pins 30 over a lead frame 10 and mask 32 supported on thebase support 22, serves to position the manifold chamber 40 in agenerally uniformly spaced relationship with respect to the base support22 and the lead frame 10, which spacing (i.e., 3/16" to 1/4") issufficient to permit the stream of abrasive particles to be exhaustedfrom the surface of the work piece. Further, the nozzle assemblies 60are in general alignment with the exposed areas of the surface of theleads 12 and the free ends of the rods 64 engage the mask 32 about thecut-out portions 34 therethrough to secure the edges thereof against thelead frame 10 and the support base 22.

When the apparatus 20 is assembled with the lead frame 10 and mask 32positioned thereon, a supply of a mixture of abrasive particles andcompressed air at a pressure of about 40 to 60 psi is fed through supplylines 52 and inlet and dispersing connectors 50 from a source thereofinto pressure chamber 54 wherein a uniform admixture in generallydesired portions of particles and gas is maintained. The pressurechamber 54 rapidly reaches the pressure of the feed gas at which timestreams of high velocity abrasive particles issue from each of thenozzle orifices 62 to impact the surface of the lead frame 10 to betreated. After a short period of time, e.g. 20 to 30 seconds, thecompressed air feed is shut off and the apparatus 20 is disassembled forremoval of the treated lead frame. I has been surprisingly found that nundesirable residue of the abrasive materials cling or are embedded inthe treated surfaces and conventional soldering techniques can then beemployed in joining the treated leads to the desire circuitry ofelectronic components and the like.

It would be evident to one skilled in the art that modifications to theapparatus using well known techniques may be made which would permit theassembly and operation thereof to be readily performed by manual orautomated means and for the process of the invention to be carried outin mass production operations.

Having thus described the invention in relation to the drawings hereof,it will be clear that modifications could be made in the preferredembodiments without departing from the spirit of the invention.Accordingly, it is not intended that the words used to describe theinvention be limiting thereof nor should the drawings be considered so.It is intended that the invention be limited only by the scope of theappended claims.

What is claimed is:
 1. An apparatus for the treatment of articles havinga gold plated surface by the high velocity impacting thereof withaparticulate abrasive material which comprises in combination:a) supportbase means for a hold plated article to be treated; b) masking meansadapted to be disposed over the article to be treated outlining at leastone selected portion of the article to be treated; and c) manifoldchamber means adaptable for receiving a quantity of particulate abrasivematerials and admixing therewith a fluid under high pressure, saidmanifold chamber means including at least one connector means foringress of a fluid under high pressure and at least one nozzle means fordispensing at high pressure and velocity fluid-borne particulateabrasive materials from within said chamber means, said manifold chambermeans further including means for engaging said masking means about saidselected portion of the article to be treated, said manifold chambermeans being movably mountable on said support means and spaced therefromwith said nozzle means and said means for engaging said masking means insubstantial register with the portion of the article to be treated. 2.The apparatus according to claim 1 including a plurality of connectormeans for ingress of fluid under high pressure.
 3. The apparatusaccording to claim 1 including a plurality of nozzle means fordispensing fluid-borne particulate abrasive materials at high pressureand velocity, said plurality of nozzle means being spaced from eachother.
 4. The apparatus according to claim 1 wherein said means forengaging said masking means is comprised of a plurality of rodsextending from said manifold chamber means.
 5. The apparatus accordingto claim 4 wherein said rods are located in the vicinity of said nozzlemeans.
 6. The apparatus according to claim 5 further including aplurality of nozzle means and wherein said rods are arranged in a seriesof arrays with each array being generally located in the vicinity ofeach nozzle means.